The need to sleep has long plagued scientists. Why do we—and in fact every other animal with a nervous system—spend such large portions of our day sleeping? After all, there are so many other aspects of life that need our attention. Studies have suggested that sleep may function to consolidate memories, help us solve difficult problems, and boost our immune system. However, there’s still no conclusive answer to why sleep is so vital.

This week, one hypothesis is gaining ground. It suggests that sleep is a “state of adaptive inactivity” that conserves energy when activity is either not required or is not particularly advantageous.

In the most recent issue of Science, a group of scientists from the Max Planck Institute for Ornithology tested this theory by looking at a system where near-constant activity, and therefore a lack of sleep, might benefit fitness. In pectoral sandpipers, male fitness is determined by access to females, and at the high Arctic latitudes in which these birds live, extremely long days enable males to engage in near-constant mating displays during periods of high female fertility. If giving up sleep to spend more time wooing potential mates increases the reproductive success of males, sleep in this species might depend more on the value of wakefulness, rather than the benefits of resting.

The researchers recorded electrical activity from the brains and the muscular systems of several male sandpipers via electroencephalogram and electromyogram dataloggers, in order to determine the amount of time the males spent sleeping each day. The sleeping patterns of males varied greatly; some males slept as little as 2.4 hours per day, while others slept more than three times as much. Males that spent the least time sleeping during periods of high female fertility were rewarded; those that slept less interacted with more females, sired young with more females, and sired greater numbers of young overall than males that slept more.

Because male pectoral sandpipers generally return to their breeding grounds yearly, it was possible for the scientists to assess survivorship by using return rates as a proxy; males that didn't return the next year were likely to have died. If sleep deprivation during the breeding season is detrimental to long-term health, males that give up sleep might be less likely to survive until the next year. However, the researchers found that males that were successful in breeding the previous year—those that slept less—were ten percent more likely to return the next year than other males. This suggests that males were not rendered less healthy over the long term by giving up sleep; in fact, they actually returned to breed at greater rates than birds that took more time out of their breeding season to sleep.

The take-home message is surprisingly simple: self-imposed sleep deprivation is positively correlated with both reproductive success and survival in male arctic sandpipers. Furthermore, this study lends some credence to the theory that sleep may not be particularly beneficial when animals have something better to do.

However, the period during which females are fertile is a relatively short three-week window. So males may be able to “catch up” on their sleep after the breeding season when females are not fertile. Additionally, the study did not test any short-term health risks of this strategy, or whether sleep deprivation in these males led to any type of reduced cognitive performance. It is possible that there are ramifications for sleep loss. But, in the grand scheme of biology, reproductive success is what matters, as it determines what traits are passed on to the next generation. And for male pectoral sandpipers, sacrificing sleep for sex appears to be a good strategy.

43 Reader Comments

Seriously though, this is such a specialized animal that trying to draw general conclusions is virtually pointless. Yes, those sand pipers who are able to pull "all dayers" have an advantage, but as noted in the article, this is for a somewhat well defined time period.

Survival rate over winter may also be explained by the same qualities that allow them to pull "all dayers", so correlation isn't causation. Again.

Or those males that are more healthy already can spend more time with females and less sleeping than those that are not that healthy. If you have sore throat (probably not very common for sandpipers) or any other illness that is not apparent to humans, than you might sleep more and have less sex.

The problem is that causality is not established: we do not know whether 1) self-imposed sleep deprivation causes reproductive success and better health, or) better health causes self-imposed sleep deprivation and reproductive success.

Or those males that are more healthy already can spend more time with females and less sleeping than those that are not that healthy. If you have sore throat (probably not very common for sandpipers) or any other illness that is not apparent to humans, than you might sleep more and have less sex.

The problem is that causality is not established: we do not know whether 1) self-imposed sleep deprivation causes reproductive success and better health, or) better health causes self-imposed sleep deprivation and reproductive success.

Yes, agreed. In fact, it is even possible that the males that sleep less during breeding season sleep less when they are away from the site too - and return more often because they are less likely to get eaten by a predator since they aren't spending much time sleeping. We really can't draw many good conclusions from the information since they didn't track the birds when they weren't on the breeding site.

So what intelligent designer was responsible for human males? Sex almost always leads to sleep, which takes one out of the running for further sex.

(And this implies that the most prodigious group will turn out to be ravers and meth addicts.)

Speak for yourself, Im neither of those and Im wide awake after doing the deed. A little exhausted mind you, but not necessarily sleepy, less so than if I hadnt had sex.

Then you, sir, are a rare person. There are chemicals released into the brain that cause relaxation & sleep in most males after sex. I think one of them is seretonin (which is also a good feeling chemical).

Wouldn't the female sandpipers choose to breed with the most fit males? Thus, the fact that a greater percentage of the "busier" birds survived is an indication that the females know how to choose a mate, rather than an indication that prodigious sex leads to greater survival? That is, the relationship is correlated, but perhaps not causal?

I like the article, but there's one thing missing: What counts as sleep-deprived for these particular birds?

There's the comment that some sleep 2.4 hours and some sleep 3x that... well I know plenty of people that sleep 6 hours and feel like a million bucks, and I know some people that can lay down and be out for 10-14 hours and still be groggy when they are up.

So... are the 2.4 hour birds sleep-deprived and fatigued and still choosing sex, or are they still fine and active on that amount of sleep?

So what intelligent designer was responsible for human males? Sex almost always leads to sleep, which takes one out of the running for further sex.

(And this implies that the most prodigious group will turn out to be ravers and meth addicts.)

The refractory period and sleepiness are actually an advantage for the human male. It gives a chance for more sperm to be produced, rather than wasting energy on a copulation that is unlikely to result in conception.

I like the article, but there's one thing missing: What counts as sleep-deprived for these particular birds?

I doubt that's well established, and probably not the term used in the paper. It may say, perhaps to the detriment of clarity, "depriving themselves of sleep" to mean a bird that chooses to pursue sex as opposed to sleep.

I really wish they would cure sleep. I spend a solid 1/4th of my life essentially dead. Cure sleep, and it is like getting that 1/4 of your life back. Screw flying cars and rockets to the moon, give me a cure for sleeping!

Except that we know people who are sleep-deprived have lowered cognitive ability - so if I'm interviewing you for a job, the simple fact that I know you like Red Bull and other artificial stimulants will automatically count against you.

Nothing personal, it's just that artificial stimulants indicate a range of other problems.

Except that we know people who are sleep-deprived have lowered cognitive ability - so if I'm interviewing you for a job, the simple fact that I know you like Red Bull and other artificial stimulants will automatically count against you.

Nothing personal, it's just that artificial stimulants indicate a range of other problems.

To counter your argument, Caffeine is proven to increase intelligence. If you wish to work for one of my companies and you do not do caffeine, that is two strikes against you.

Except that we know people who are sleep-deprived have lowered cognitive ability - so if I'm interviewing you for a job, the simple fact that I know you like Red Bull and other artificial stimulants will automatically count against you.

Nothing personal, it's just that artificial stimulants indicate a range of other problems.

Furthermore, this study lends some credence to the theory that sleep may not be particularly beneficial when animals have something better to do.

Aw bullshit. Not that conserving energy will not be part of it. No other reason for big cats to sleep through most of their days. But it alone does not explain why more or less all mammals and birds will die a pretty gruesome death if we impose constant sleeplessness over a long period of time.

What would be the benefit of DYING because of sleeplessness if all it is is energy conservation. Those guys need a jump to conclusions mat.

So what intelligent designer was responsible for human males? Sex almost always leads to sleep, which takes one out of the running for further sex.

(And this implies that the most prodigious group will turn out to be ravers and meth addicts.)

Speak for yourself, Im neither of those and Im wide awake after doing the deed. A little exhausted mind you, but not necessarily sleepy, less so than if I hadnt had sex.

Then you, sir, are a rare person. There are chemicals released into the brain that cause relaxation & sleep in most males after sex. I think one of them is seretonin (which is also a good feeling chemical).

And females too I must say. I'm surprised at how fast girls fall asleep right after I've had an intimate moment with them. So I think there is some sort of chemical released in females as well that make them fall asleep.

So what intelligent designer was responsible for human males? Sex almost always leads to sleep, which takes one out of the running for further sex.

(And this implies that the most prodigious group will turn out to be ravers and meth addicts.)

Speak for yourself, Im neither of those and Im wide awake after doing the deed. A little exhausted mind you, but not necessarily sleepy, less so than if I hadnt had sex.

Then you, sir, are a rare person. There are chemicals released into the brain that cause relaxation & sleep in most males after sex. I think one of them is seretonin (which is also a good feeling chemical).

And females too I must say. I'm surprised at how fast girls fall asleep right after I've had an intimate moment with them. So I think there is some sort of chemical released in females as well that make them fall asleep.

Not just after. If the rumors about you, all over the internet, in 16 languages are true, quite often during. </joke>

Then you, sir, are a rare person. There are chemicals released into the brain that cause relaxation & sleep in most males after sex. I think one of them is seretonin (which is also a good feeling chemical).

Except that we know people who are sleep-deprived have lowered cognitive ability - so if I'm interviewing you for a job, the simple fact that I know you like Red Bull and other artificial stimulants will automatically count against you.

Nothing personal, it's just that artificial stimulants indicate a range of other problems.

So what intelligent designer was responsible for human males? Sex almost always leads to sleep, which takes one out of the running for further sex.

(And this implies that the most prodigious group will turn out to be ravers and meth addicts.)

Speak for yourself, Im neither of those and Im wide awake after doing the deed. A little exhausted mind you, but not necessarily sleepy, less so than if I hadnt had sex.

Then you, sir, are a rare person. There are chemicals released into the brain that cause relaxation & sleep in most males after sex. I think one of them is seretonin (which is also a good feeling chemical).

And females too I must say. I'm surprised at how fast girls fall asleep right after I've had an intimate moment with them. So I think there is some sort of chemical released in females as well that make them fall asleep.

Not just after. If the rumors about you, all over the internet, in 16 languages are true, quite often during. </joke>

I really wish they would cure sleep. I spend a solid 1/4th of my life essentially dead. Cure sleep, and it is like getting that 1/4 of your life back. Screw flying cars and rockets to the moon, give me a cure for sleeping!

This reminded me of a drug I read about years ago which the military was working with which allowed solders to stay awake long periods of time without side effects. So I looked it up. The results were kind of disappointing. According to the articles I read, it seems 40 hours is the limit and you still have to "catch up" on your sleep later. On the good side, loss in cognitive ability and alertness during the 40 hours was significantly reduced compared to staying up without the drug.

I really wish they would cure sleep. I spend a solid 1/4th of my life essentially dead. Cure sleep, and it is like getting that 1/4 of your life back. Screw flying cars and rockets to the moon, give me a cure for sleeping!

I wish they would cure sleep and never let it be known the public. As soon as the cure of sleep becomes common knowledge, I guarantee you can watch your wages go down and your work hours go up.

I'll keep up pretense of satisfactorily getting enough sleep in return for having that dedicated free time, thank you very much!

This was a concise study that was arguably overly narrow, that will be taken way out of context and cited in an overly broad manner.

Study TL;DR: Sandpipers in rut are more successful if they sleep less, and appear to be better at survival.

:GOLFCLAP: I applaud them for confirming something which Darwin predicted. Tolerance for longer periods of alertness usually helps with individual survival, and natural selection would tend to favor those who are awake during rut.

But we have stacks and stacks of evidence that sleep deprivation affects humans, and thanks to our much better capacity to communicate with each other then with sandpipers, and to catalog and measure our cognitive faculties in various states of alertness, we can be very confident about this conclusion:

Humans... need... sleep; and we generally seem to need more then sandpipers. Even if we don't completely understand why.

They really screwed up the controls on this study. They needed to look at RELATIVE sleep levels of individual sandpipers with respect to their normal habits, not just absolute sleep numbers. It could be that the ones that were getting by on less sleep have a mutation that allows them to get by on significantly less sleep, while the ones that slept longer were punishing themselves trying to keep up. If that's the case, then it could still be that sleeping less than one's individual needs will increase risk of death. The language used in the article seems to imply that going with less sleep improves health in this species, which I don't think is likely, and definitely not proven with the experiment as it was performed.

Kate Shaw Yoshida / Kate is a science writer for Ars Technica. She recently earned a dual Ph.D. in Zoology and Ecology, Evolutionary Biology and Behavior from Michigan State University, studying the social behavior of wild spotted hyenas.